1. Field of the Invention
The present invention relates to a nitrogen oxide (and more, sometimes abbreviated as “NOx”) purification catalyst, more particularly relates to a novel NOx purification catalyst comprised of particles including gold atoms and iron atoms and able to purify NOx at a low temperature and/or in an oxidizing atmosphere.
2. Description of the Related Art
In recent years, from the viewpoint of protection of the global environment, exhaust gas regulations have been toughened around the world with each passing year. As one means for dealing with this, in internal combustion engines, exhaust gas purification catalysts are being used. In such exhaust gas purification catalysts, in order to efficiently remove HC (hydrocarbons), carbon monoxide (CO), and NOx in the exhaust gas, platinum, gold, rhodium, and other precious metals have been used as catalyst ingredients.
In motor vehicles using such purification catalysts, for example gasoline engine vehicles and diesel engine vehicles, various systems are being used to improve the catalytic activity and the fuel economy. For example, for improving fuel economy during steady state operation fuel is burned under conditions of a lean (excess oxygen) air-fuel ratio (A/F), while to improve the catalytic activity, fuel is burned under temporarily stoichiometric (stoichiometric air-fuel ratio, A/F=14.7) to rich (excess fuel) conditions.
These conditions apply because conventionally known platinum, gold, rhodium, and other precious metal catalysts exhibit low NOx purification performances at low temperature and under oxidizing conditions. To improve the purification performance, the purification catalyst has to be operated high in temperature and HC or CO etc. has presently to be added to establish a reducing atmosphere. Therefore, even during steady state operation, it is not possible to increase the air-fuel ratio (A/F).
With the above precious metal catalysts, there is therefore a limit to the improvement of the fuel economy. In such conventionally known precious metal catalysts, to obtain purification performance, energy for raising the purification catalysts to a high temperature, fuel for temporarily exposing the purification catalysts to a reducing atmosphere, and reduction of the air-fuel ratio (A/F) in the engine are all required. To improve the fuel economy in automobile engines and other internal combustion engines, therefore, a new purification catalyst able to exhibit a good NOx purification performance at a low temperature and/or in an oxidizing atmosphere is being sought.
Furthermore, all of the above precious metal catalysts face the problem of resource depletion. NOx purification catalysts using other metals to obtain purification performances equal to or better than those of conventional precious metal catalysts or able to reduce the amounts of the precious metals used are therefore being sought.
For example, Japanese Patent Publication (A) No. 10-216518 describes a gold alloy catalyst comprised of gold and one or more metals (M) selected from platinum, palladium, silver, copper, and nickel, having a weight ratio Au/M of 1/9 to 9/1, and having an amount of solute gold in the alloy of 20 to 80 wt %. Further, the catalysts shown as specific examples in this publication are catalysts supporting a gold alloy of gold and a metal of palladium or platinum on an Al2O3 carrier. These exhibit a high NOx purification performance in a reducing atmosphere, but have a low NOx purification performance at a low temperature and/or in an oxidizing atmosphere.
Further, Japanese Patent Publication (A) No. 10-216519 describes a metal particulate-carrying oxide catalyst produced by heat treating a metal compound composed of one or more elements selected from gold, platinum, palladium, rhodium, silver, iridium, ruthenium, and osmium and one or more elements selected from metals of strontium, niobium, lithium, lanthanum, aluminum, silicon, magnesium, calcium, barium, cerium, neodymium, titanium, iron, cobalt, nickel, copper, zinc, zirconium, vanadium, tantalum, chromium, molybdenum, tungsten, sodium, potassium, beryllium, scandium, yttrium, indium, tin, lead, and bismuth in an atmosphere containing oxygen. Further, the catalysts shown as specific examples in this publication are catalysts composed of gold alloy incorporating strontium or lanthanum on an Al2O3 carrier. They exhibit a relatively high NOx purification performance in a reducing atmosphere, but have an extremely low NOx purification performance at a low temperature and/or in an oxidizing atmosphere.
Further, Japanese Patent Publication (A) No. 2001-239161 describes a low temperature harmful gas-purification catalyst comprised of a metal oxide on carbonaceous material carrier using high temperature/high pressure fluid to carry ultrafine particles of at least one type of metal selected from the group comprising platinum, palladium, rhodium, ruthenium, iridium, osmium, gold, silver, copper, manganese, iron, and nickel. Further, the catalysts shown as specific examples in this publication are purification catalysts supporting one of type of platinum, palladium, rhodium, ruthenium, nickel, or gold. They exhibit a good NOx purification performance in a reducing atmosphere.
Furthermore, Japanese Patent Publication (A) No. 2003-190787 describes an engine exhaust gas purification catalyst mainly comprised of 12CaO.7Al2O3 and carrying one or more elements selected from gold, silver, iron, zinc, manganese, cerium, and platinum group elements. Further, this publication shows, as specific examples, that purification catalysts mainly comprised of 12CaO.7Al2O3 carrying one element out of gold, silver, platinum, palladium, copper, iron, zinc, manganese, cerium, or rhodium or two elements out of silver and rhodium, ruthenium, or copper fired at 600° C. exhibit the effect of reducing the combustion temperature by the oxidation reaction of the particulate matter (PM) by oxygen radicals. However, this publication does not prescribe the positional relationship of the two types of metals. It is unclear if the catalysts shown as specific examples in this publication exhibit an NOx purification performance.
As related art, therefore, there are the above Japanese Patent Publication (A) No. 10-216518, Japanese Patent Publication (A) No. 10-216519, Japanese Patent Publication (A) No. 2001-239161, and Japanese Patent Publication (A) No. 2003-190787.